A surveillance system using an array of detectors onto which the image of a scene under surveillance is focused can locate objects in direction but not absolutely in position, as only the angle at which the energy enters the optical system corresponding to a given array element is defined. Even if the array is of thermal detectors, an attempt to calculate the distance of an object from the array by absolutely measuring the quantity of radiation falling on an element is subject to major uncertainties such as the size, temperature and emissivity of the object detected. A particular case where the absolute position of the object would be of value is in unattended surveillance systems using arrays of pyroelectric elements utilising unchopped infrared radiation, where information about the location and path of an intruder can be used to facilitate his arrest.